The use of dehydration or drying processes to reduce or adjust the moisture content of, especially, organic materials has been practiced for many years. The purposes of dehydration have included the preservation of food products, the curing of wood and other construction materials, the drying of clothing and other fabrics, and the preservation of flowers and other decorative plant materials. In the original and most basic forms, dehydration is achieved by handing the materials to be dried on a simple rack and exposing them to sunlight, but the dependence of that approach on uncontrollable environmental factors creates significant disadvantages and has lead to the development of more sophisticated approaches, including the circulation of air in an enclosure containing the materials to be dried.
In one air circulation approach to dehydration, warmed air is circulated through the materials to be dried to remove moisture, and is then exhausted from the system, as generally illustrated by U.S. Pat. No. 4,241,515 to Wochnowski et al., which discloses a method and apparatus for conditioning tobacco. This approach represents an improvement over the use of ambient air for drying, but does not provide any means for dehumidification of the air used for drying and thus does not allow independent control of both air temperature and moisture content.
U.S. Pat. No. 4,447,965 to Bray discloses a drying process and apparatus which dehumidifies a vapor laden effluent by condensing the vapor through acceleration of the effluent stream in a venturi-type device. The dried air, or other drying fluid, is then compressed, heated, and recirculated. This approach is generally effective for moisture removal, but is subject to several significant limitations and disadvantages. Proper operation of the venturi-type device requires that parameters such as air flow, pressure, and temperature be maintained within narrow ranges, and the apparatus is necessarily designed without any means of independent control of those, and other parameters. As a result, this approach cannot be effectively used when it is necessary or desirable to vary the conditions of the stream of air through the dehydration apparatus.
U.S. Pat. No. 4,250,629 to Lewis illustrates an approach In another approach to dehydration moisture laden air is passed over cooling coils to condense and remove moisture from the air stream, as illustrated by U.S. Pat. Nos. 4,250,629 to Lewis and 3,931,683 to Crites, et al. In the Lewis and Crites examples the air is then heated and circulated around the material to be dried. The apparatus disclosed by both examples utilize a compression-type refrigeration system with evaporator coils and condenser coils to respectively cool and heat the air stream. The Lewis apparatus is specifically intended for use as a lumber conditioning kiln, and the Crites apparatus is intended for drying granular-type materials such as cereal grains. The Lewis apparatus requires the inclusion of a by-pass air passage to recirculate air without passing over the cooling coils or heating coils in certain circumstances, for the primary purpose of controlling the operating parameters of the refrigeration system rather than precisely controlling the parameters of the stream of air passing over the lumber to be dried. The lack of precise control drying air conditions does not present a problem for the conditioning of lumber, but is a serious disadvantage for the dehydration of more delicate materials, such as flowers. The Crites system provides direct contact between the material to be dried and the heat exchange surfaces of the heating coils, an approach which is efficient for the drying of grains but unsuitable for delicate materials, and does not provide for precise control of either the temperature or the humidity of the air stream.
There remains a need for a dehydration apparatus and process that allows precise and efficient control of the parameters of the stream of air or other drying fluid, such as temperature, humidity, and velocity, during the dehydration operation. There is also a need for a dehydration apparatus and process which allows those parameters to be varied during the dehydration operation as needed for optimal efficiency and product quality.